How do I enter on QB a cash amount that I gave for Christmas bonus to employees so it reflects on their next paystub?

Let me assist you on how to enter your Christmas bonus for your employees, drjohnson.

In QuickBooks, you can pay your employees with bonuses along with their regular pay. Please know that the bonus amount is taxed the same way as the employee's salary. 

Before you type the amount of the bonus of your employee, you'll have to put a checkmark in the Bonus option first and then manually enter the bonus rate when running a paycheck.

Follow these steps: 

Step 1: Tick the Bonus option.

1. Go to the Payroll menu then click the Employees tab. 
2. Select an employee. 
3. Scroll down and look for the Pay Types section. Then hit Edit. 
4. In the Common Pay Types menu, tick the Bonus. 
5. Press Save. 
    

Step 2: Manually enter the bonus rate of your worker. 

1. Under the Employees tab, click Run Payroll.
2. Choose a pay schedule for the said payroll and then press Continue.
3. In the Run Payroll window, tick the name of your employee. 
4. You can enter the number by clicking on the Bonus column. Ensure that it aligns with the name of your worker.
5. After typing the amount, you can either click Save for Later or Preview Payroll.

I'll provide you with this material in case you want to know more about how to pay an employee bonus using QuickBooks Payroll: Pay an employee bonus. 

You can also add overtime pay to an employee to pay them for their extra hours.

If you have further questions regarding QuickBooks Online Payroll, you can type your concern in the comment section. We're always here to help you.

@IrizA  The OP did not ask how to set up and  use bonus items. You did not answer the question.

The question is: "How do I enter on QB a cash amount that I gave for Christmas bonus to employees so it reflects on their next paystub"

The question does include the word bonus, but that is about as close to your answer as it gets.  Did you even read the question?